Not applicable
1. Field of the Invention
This invention relates generally to hard disk drive memory storage devices and more particularly to a method and apparatus for determining and correcting the absolute track spacing without having to traverse across the entire disk surface.
2. Description of the Related Art
As described in International Patent Application, WO 94/11864, high track densities are possible with voice-coil and other types of servo positioners as well as the ability to read and write narrower tracks by using, for example, magneto resistive (MR) head technology. Previously, low track density disk drives were able to achieve satisfactory head positioning with lead screw and stepper motor mechanisms. However, when track densities are so great that the mechanical error of a lead screw-stepper motor combination is significant compared to track-to-track spacing, an embedded servo is needed so that the position of the head can be determined from the signals it reads.
Conventional hard disk manufacturing techniques including writing servotracks on the media of a head disk assembly (HDA) with a specialized servowriter instrument. Laser positioning feedback is used in such instruments to read the actual physical position of a recording head used to write the servotracks. Unfortunately, it is becoming more and more difficult for such servowriters to invade the internal environment of a HDA for servowriting because the HDAs themselves are exceedingly small and depend on their covers and castings to be in place for proper operation. Some HDAs are the size and thickness of a plastic credit card. At such levels of micro miniaturization, traditional servowriting methods are inadequate.
Conventional servo-patterns typically comprise short bursts of a constant frequency signal, very precisely located offset from a data track""s center line, on either side. The bursts are written in a sector header area, and can be used to find the center line of a track. Staying on center is required during both reading and writing. Since there can be between seventeen to sixty, or even more, sectors per track, that same number of servo data areas must be dispersed around a data track. These servo-data areas allow a head to follow a track center line around a disk, even when the track is out of round, as can occur with spindle wobble, disk slip and/or thermal expansion. As technology advances provide smaller disk drives, and increased track densities, the placement of servo data must also be proportionately more accurate.
Servo-data is conventionally written by dedicated, external servowriting equipment, and typically involves the use of large granite blocks to support the disk drive and quiet outside vibration effects. An auxiliary clock head is inserted onto the surface of the recording disk and is used to write a reference timing pattern. An external head/arm positioner with a very accurate lead screw and a laser displacement measurement device for positional feedback is used to precisely determine transducer location and is the basis for track placement and track-to-track spacing. The servo writer requires a clean room environment, as the disk and heads will be exposed to the environment to allow the access of the external head and actuator.
U.S. Pat. No. 4,414,589 to Oliver et al. issued Dec. 14, 1981, entitled xe2x80x9cServo track following system and method for writing servo tracksxe2x80x9d discloses servowriting wherein optimum track spacing is determined by positioning one of the moving read/write heads at a first limit stop in the range of travel of the positioning means. A first reference track is then written with the moving head. A predetermined reduction number or percentage of amplitude reduction X %, is then chosen that is empirically related to the desired average track density. The first reference track is then read with the moving head. The moving head is then displaced away from the first limit stop until the amplitude of the first reference track is reduced to X % of its original amplitude. A second reference track is then written with the moving head and the moving head is then displaced again in the same direction until the amplitude of the second reference track is reduced to X % of its original value. The process is continued, writing successive reference tracks and displacing the moving head by an amount sufficient to reduce the amplitude to X % of its original value, until the disc is filled with reference tracks. The number of reference tracks so written is counted and the process is stopped when a second limit stop in the range of travel of the positioning means is encountered. Knowing the number of tracks written and the length of travel of the moving head, the average track density is checked to insure that it is within a predetermined range of the desired average track density. If the average track density is high, the disc is erased, the X % value is lowered and the process is repeated. If the average track density is low, the disc is erased, the X % value is increased and the process is repeated. If the average track density is within the predetermined range of the desired average track density, the desired reduction rate X %, for a given average track density, has been determined and the servo writer may then proceed to the servo writing steps.
Unfortunately, Oliver et al. does not disclose how to determine absolute spacing without first servo writing the entire hard disk surface. The servowriter throughput is substantially reduced by having to rewrite the entire surface whenever the spacing is outside the acceptable range. Accordingly, a need exists to determine and adjust the absolute accuracy of the servo pattern without the need to rewrite the entire surface whenever the spacing is outside an acceptable tolerance.
U.S. Pat. No. 5,612,833 of Yarmchuk et al, issued Mar. 18, 1997 entitled xe2x80x9cRadial Self-propagation Pattern Generation For Disk File Servowritingxe2x80x9d discloses a method and apparatus for radial self-propagation pattern generation, and commonly assigned herewith. Yarmchuk disclosed a method and apparatus of writing more uniformly spaced tracks. During that write revolution, a position error signal corresponding to the position error of the transducer relative to the previously recorded transition is determined. That position error signal is then stored, during the write revolution, to be used in computing a reference track value associated with the transition being written to correct for the position error. Additionally, a product servo-pattern is written, which includes an embodying of the position error therein.
A shortcoming of the some prior techniques is the lesser accuracy in placement of the servo patterns. The requirement for ever closer track spacing in disk files makes highly accurate servo pattern writing highly desired. Therefore a need exists for a method to accurately determine and adjust the track spacing of a self-servowritten disk without adding any additional equipment, either externally or internally, to the disk unit in order to measure the absolute pattern spacing.
Many newer disk drive units are now designed with a load/unload ramp at the outer edge of the disk instead of a rigid crash stop at the outer crash stop. The head suspension is lifted away from the disk surface by this ramp during the process of unloading the head from the disk. Unlike a rigid crash stop, the actuator arm can be moved beyond the point of contact. This makes detection more difficult, and can lead to disk damage if the heads are only partially lifted off the disk. The unload process is highly reliable, but must take place in one continuous motion. Therefore, a need exists for determining and correcting the absolute track spacing of a self-servowritten disk without having to rely on a crash stop.
Briefly, in accordance with the present invention, disclosed is a method and apparatus to determine and correct track spacing during self-servowriting on a rotating recording medium. The recording medium comprising a plurality of tracks, wherein each track comprises a plurality of sectors, and a transducer mounted on an actuator arm pivotally coupled to a voice coil motor (VCM). The actuator arm is positioned by a servo. The method comprising the steps of: servowriting the at least one of the plurality of sectors with a servo pattern consisting of recorded transitions. The servowriting is performed on one more tracks within the sectors where the number of tracks being servowritten is less than the total number of tracks that fills the rotating medium. The transducer is positioned relative to the rotating recording medium to a preselected radial position over a previously servowritten area of the rotating recording medium that has one or more previously recorded transitions. Next, an angular acceleration is imposed on the actuator arm by applying a predetermined amount of current to the VCM. The measurement and correction of a spacing of the tracks in the previously servowritten area is performed by measuring the amplitudes of the previously recorded transitions at least one time during the passage of the sectors beneath the transducer, and if the calibrating of the spacing is outside a predetermined tolerance, then continuing servowriting new recorded transitions using said adjustment fact on tracks following said previously servowritten area. In one embodiment, the method includes measuring a VCM torque constant (K) by applying a current impulse for a predetermined time (t) and measuring the back Electromotive Force (EMF) generated from the VCM to determine the torque per unit for the current impulse for the predetermined time (t) and to determine the back Electromotive Force (EMF) per unit of angular velocity of the actuator arm. After the torque constant is determined, an adjustment factor is computed based on the values of the torque constant (K), the current impulse for the period of time (t), and the back Electromotive Force (EMF) per unit of angular velocity of the actuator arm. This adjustment factor is used while servowriting new recorded transitions tracks following the previously servowritten area.